1. Field of the Invention
The present invention relates to a display device including a display unit having a light emitting element and a pixel circuit for each of pixels, and a drive unit for driving the pixel circuit, and to a method for driving the same. The present invention also relates to an electronic device having the display device.
2. Description of the Related Art
In recent years, in the field of a display device for displaying an image, a display device using, as a light emitting element of a pixel, an optical element of a current driving type whose light emission luminance changes according to a value of a flowing current, for example, an organic EL (Electro Luminance) element, is developed and is being commercialized.
The organic EL element is a spontaneous light emitting element unlike a liquid crystal element or the like. Thus, in a display device using an organic EL element (organic EL display device), a light source (backlight) is unnecessary. Therefore, as compared with a liquid crystal display device necessitating a light source, visibility of an image is higher, power consumption is lower, and response of the element is faster.
As driving methods of the organic EL display device, as in the liquid crystal display device, there are a simple (passive) matrix method and an active matrix method. The simple (passive) matrix method has, although a structure is simple, a disadvantage in that a large-sized high-resolution display device is difficult to be realized. Consequently, at present, the active matrix method is actively developed. In the active matrix method, current flowing in a light emitting element disposed for each pixel is controlled by an active element (generally, TFT (Thin Film Transistor)) provided in a drive circuit arranged for each of the light emitting elements.
Generally, a current-voltage (I-V) characteristic of the organic EL element deteriorates with time (time-dependent degradation). In a pixel circuit for current-driving the organic EL element, when the I-V characteristic of the organic EL element changes with time, a voltage dividing ratio between the organic EL element and a drive transistor connected in series with the organic EL element changes, so that a voltage Vgs between a gate and a source of the drive transistor also changes. As a result, a value of current flowing in the drive transistor changes, so that a value of current flowing in the organic EL element also changes, and light emission luminance also changes according to the current value.
In addition, there is a case that a threshold voltage Vth and mobility μ of the drive transistor change with time, or differ among the pixel circuits due to variations in manufacturing processes. In a case where the threshold voltage Vth and mobility μ of the drive transistor differ among the pixel circuits, the value of current flowing in the drive transistor varies among pixel circuits. Consequently, even when the same voltage is applied to the gate of the drive transistor, the light emission luminance of the organic EL element varies, and uniformity of a screen deteriorates.
Accordingly, a display device is developed, which has a function of compensating fluctuations in the I-V characteristic of the organic EL element and a function of correcting fluctuations in the threshold voltage Vth and the mobility μ of the drive transistor, in order to maintain the light emission luminance of the organic EL element without being influenced by the variations with time in the I-V characteristic of the organic EL element and the variations with time in the threshold voltage Vth and the mobility μ of the drive transistor (see, for example, Japanese Unexamined Patent Application Publication No. 2008-083272).
FIG. 10 illustrates a schematic configuration of a display device described in JP2008-083272A. A display device 100 illustrated in FIG. 10 has a display unit 110 in which a plurality of pixels 120 are disposed in a matrix, and a drive unit (a horizontal drive circuit 130, a write scan circuit 140, and a power source scan circuit 150) for driving each of the pixels 120.
Each of the pixels 120 includes a pixel 120R for red, a pixel 120G for green, and a pixel 120B for blue. As illustrated in FIG. 11, each of the pixels 120R, 120G, and 120B includes an organic EL element 121 (organic EL elements 121R, 121G, and 121B) and a pixel circuit 122 connected to the organic EL element 121. The pixel circuit 122 includes a transistor TWS for sampling, a retention capacitor Cs, and a transistor TDr for driving, and has a circuit configuration of 2Tr1C. A gate line WSL led from the write scan circuit 140 is formed to extend in a row direction and is connected to a gate of the transistor TWS. A drain line DSL led from the power source scan circuit 150 is also formed to extend in the row direction, and is connected to a drain of the transistor TDr. A signal line DTL led from the horizontal drive circuit 130 is formed to extend in a column direction, and is connected to a drain of the transistor TWS. A source of the transistor TWS is connected to a gate of the transistor TDr for driving and to one end of the retention capacitor Cs. A source of the transistor TDr and the other end of the retention capacitor Cs are connected to an anode of the organic EL element 121R, 121G, or 121B (hereinbelow, simply referred to as an “organic EL element 121R or the like”). A cathode of the organic EL element 121R or the like is connected to a ground line GND.
FIG. 12 represents an example of various waveforms in the display device 100 illustrated in FIG. 10. FIG. 12 represents a state where two kinds of voltages (Von and Voff (<Von)) are applied to the gate line WSL, two kinds of voltages (Vcc and Vini (<Vcc)) are applied to the drain line DSL, and two kinds of voltages (Vsig and Vofs (<Vsig)) are applied to the signal line DTL. Further, FIG. 12 represents a state where a gate voltage Vg and a source voltage Vs of the transistor TDr change momentarily in accordance with application of the voltages to the gate line WSL, the drain line DSL and the signal line DTL.
[Vth Correction Preparation Period]
First, Vth correction is prepared. Specifically, the power source scan circuit 150 decreases the voltage of the drain line DSL from Vcc to Vini (T1). As a result, the source voltage Vs decreases to Vini, and light of the organic EL element 121 or the like goes out. At this time, the gate voltage Vg also decreases due to coupling via the retention capacitor Cs. Next, while the voltage of the signal line DTL is Vofs, the write scan circuit 140 increases the voltage of the gate line WSL from Voff to Von (T2). As a result, the gate voltage Vg decreases to Vofs.
[First Vth Correction Period]
Next, Vth is corrected. Specifically, while the voltage of the signal line DTL is Vofs, the power source scan circuit 150 increases the voltage of the drain line DSL from Vini to Vcc (T3). As a result, current Ids flows between the drain and the source of the transistor TDr, and the source voltage Vs rises. After that, before the horizontal drive circuit 130 switches the voltage of the signal line DTL from Vofs to Vsig, the write scan circuit 140 decreases the voltage of the gate line WSL from Von to Voff (T4). As a result, the gate of the transistor TDr floats, and correction of Vth is temporarily stopped.
[First Vth Correction Stop Period]
In a period in which the Vth correction is stopped, the voltage of the signal line DTL is sampled in another row (pixel) different from a row (pixel) in which the Vth correction is performed. In a case where the Vth correction is insufficient, that is, in the case where a potential difference Vgs between the gate and the source of the transistor TDr is larger than the threshold voltage Vth of the transistor TDr, the current Ids flows between the drain and the source of the transistor TDr and thus the source voltage Vs rises also in the Vth correction stop period in the row (pixel) in which the Vth correction is performed earlier, and the gate voltage Vg also rises by the coupling via the retention capacitor Cs.
[Second Vth Correction Period]
After completion of the Vth correction stop period, Vth is corrected again. Specifically, when the voltage of the signal line DTL is Vofs and Vth correction is possible, the write scan circuit 140 increases the voltage of the gate line WSL from Voff to Von (T5) and connects the gate of the transistor TDr to the signal line DTL. At this time, in a case where the source voltage Vs is lower than Vofs−Vth (in the case where the Vth correction has not been completed), the current Ids flows between the drain and the source of the transistor TDr until the transistor TDr cuts off (until the voltage difference Vgs becomes Vth). As a result, the retention capacitor Cs is charged to Vth, and the potential difference Vgs becomes Vth. After that, before the horizontal drive circuit 130 switches the voltage of the signal line DTL from Vofs to Vsig, the write scan circuit 140 decreases the voltage of the gate line WSL from Von to Voff (T6). As a result, the gate of the transistor TDr floats so that the potential difference Vgs is maintained at Vth irrespective of the magnitude of the voltage of the signal line DTL. In this way, by setting the potential difference Vgs to Vth, light emission luminance of the organic EL elements 121 or the like is prevented from varying even when the threshold voltage Vth of the transistor TDr is varied among the pixel circuits 122.
[Second Vth Correction Stop Period]
Thereafter, in the Vth correction stop period, the horizontal drive circuit 130 switches the voltage of the signal line DTL from Vofs to Vsig.
[Write and μ Correction Period]
After completion of the Vth correction stop period, writing and μ correction are performed. Specifically, while the voltage of the signal line DTL is Vsig, the write scan circuit 140 increases the voltage of the gate line WSL from Voff to Von (T7) and connects the gate of the transistor TDr to the signal line DTL. As a result, the voltage of the gate of the transistor TDr becomes Vsig. At this time, the voltage of the anode of the organic EL element 121R or the like is smaller than threshold voltage Vel of the organic EL element 121R or the like at this stage, and the organic EL element 121R or the like is cut off. Consequently, the current Ids flows to an element capacitor (not illustrated) of the organic EL element 121R or the like, and the element capacitor is charged. Thus, the source voltage Vs rises by ΔV, and eventually the potential difference Vgs becomes Vsig+Vth−ΔV. In this way, the μ correction is performed at the same time with the writing. Here, the larger the mobility μ of the transistor TDr is, the larger ΔV becomes. Therefore, by decreasing the potential difference Vgs by ΔV before the light emission, the variations in the mobility μ per pixel is eliminated.
[Light Emission]
Finally, the write scan circuit 140 decreases the voltage of the gate line WSL from Von to Voff (T8). As a result, the gate of the transistor TDr floats, the current Ids flows between the drain and the source of the transistor TDr, and the source voltage Vs rises. Consequently, the organic EL element 121R or the like emits light with desired luminance.